A perennial problem in the design of diagnostic ultrasound systems is that of realizing relatively optimal spacial resolution over the entire depth range of the image plane. Thus, while individual approaches to design of ultrasound imaging systems dictate different compromises in design, operation, or ultimate image quality, the underlying difficulty is never far from the surface.
One well known class of ultrasound imaging system is that which utilizes a transducer element and an associated acoustic lens for generation of sonic transmissions, and receipt of sonic echoes, based upon which the image is constructed. See, for example, U.S. Pat. Nos. 4,131,021 and 4,131,022 to Mezrich et al., and an extensive series of improvement patents. In one version of system in this class, which has found greatest commercial favor, the transducer and lens both are circular in configuration and spaced apart, the lens is stationary, and the transducer is oscillated or "nodded" through predetermined arc. Transmission and receipt of echo pulses occurs at predetermined increments of the arc, and the composite of the individual echoes is a B-scan.
In accordance with the transducer-lens approach to ultrasound design, the convergence of the sonic beam results in a zone of focus at which the beam is narrowest, and on both sides of which it gradually diverges. The outer points of the "good focus" zone are of course subjectively defined, depending upon the nature of the imaging being conducted, and the corresponding tolerance of relatively poorer spacial resolution at the top and bottom of the image field. Parenthetically, it is to be noted that the same condition obtains in those systems wherein a lenslike focusing element is attached directly to the front of a flat transducer element, or wherein the transducer itself has a curved or shaped beam focusing surface.
One prior art approach to expansion of the high resolution segment of the image field has been to divide the planar transducer into a multiple ring concentric annular array, each ring of which, in association with the lens, has a different optimal focal characteristic. In the aggregate, there results an extended field of well focused ultrasound energy. Annular array transducers, however, are quite difficult to design and fabricate, and are inherently structurally complex; they also require complex and expensive support electronics, and, as of the current state of the art, are notoriously unreliable. Moreover, increases in the multiplicity of annular ring elements result in corresponding increases in supporting cable connections and the like. This sheer physical bulk mitigates against elegant and reliable transducer oscillation mechanisms, connections, and designs. Finally, empirical design considerations for annular array systems often entail compromises which substantially reduce the high resolution field depth capability from that which is theoretically available.
It is accordingly a primary object of the present invention to provide ultrasound scanning systems, and especially those of the transducer and lens type, which present extended zones of acceptable focus and hence of satisfactory spatial resolution, while avoiding annular transducer arrays and the difficulties inherent in those schemes.